Crowds thronged into a new freshwater aquarium at Ocean Park yesterday to catch their first glimpse of 10 Chinese sturgeon, more than a year after several in the first batch of the rare species sent to the marine park died or fell sick.

The new freshwater aquarium is a departure from the saltwater environment that greeted the first batch of Chinese sturgeon in 2008. The fish, native to the Yangtze River, are born in fresh water but swim out to sea before returning to fresh water to breed. Sturgeon are kept in fresh water on the mainland.

Ocean Park chairman Allan Zeman said the 10 Chinese sturgeon seemed to do better in their new freshwater environment. The original Chinese sturgeon aquarium was converted from a shark aquarium and had had a private area for the fish since August 2008, when one of the sturgeon was killed by a barracuda.

'So far, they are doing very well,' Zeman said. 'As they get bigger, we'll release some of them.'

Zeman said there were currently no plans to breed them in the aquarium. Female sturgeon can lay between 300,000 and 1.3 million eggs at a time, but usually only about 10 per cent survive.

In April, the National Aquatic Wildlife Conservation Association released 108 Chinese sturgeon into the Jiulongjiang estuary in Xiamen . Four of them were from the first batch on display at Ocean Park.

The fish make the adjustment from freshwater to saltwater environments differently. In the wild, this means different Chinese sturgeon may need to swim to shallower or deeper water to acclimatise to the level of salt in the water. But in an enclosed saltwater aquarium, this is not possible, deputy chief executive Matthias Li Shing-chung said.

The 10 Chinese sturgeon on display include three 10-year-old fish that the marine park sent back earlier to the mainland.

The sturgeon were quarantined for a month after they arrived in Hong Kong last month. The longest measures 2.2 metres, and the species can grow to five metres.

The new attraction, which opened just two weeks before the end of Ocean Park's fiscal year on June 30, is expected to provide a boost to attendance.

With a 27 per cent year-on-year jump in attendance in May to a new record for that month and strong results for June so far, Zeman said the theme park's management had high hopes for a record-breaking year if the fine weather continued.

The number of Chinese sturgeon on display at Ocean Park: 10

Females lay up to 1.3 million eggs, but the proportion that survive is about: 10%